Isolate console-sensitive tests from concurrent logging

[Widthdom]

Summary

• mark the shared console-sensitive xUnit collection as non-parallel
• enroll every remaining process-global console capture owner in that collection
• pin the collection assignments with a reflection regression contract
• document the isolation rule in both English and Japanese

Root cause

ConsoleCapture serialized writer replacement through ConsoleStreamOwnership.Gate, but the console-sensitive xUnit collection itself could still run beside independent test classes. An unrelated MCP request-id or global diagnostic write could therefore land in the temporarily captured process-wide stderr stream.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release --framework net8.0 --no-build -p:UseSharedCompilation=false — 10,373 passed, 7 skipped, 0 failed
• final latest-main sync: net8.0 console/issue filter — 9 passed
• final latest-main sync: net9.0 console filter — 8 passed
• dotnet build CodeIndex.sln -c Release --no-restore -p:UseSharedCompilation=false — 0 warnings, 0 errors
• dotnet format CodeIndex.sln --verify-no-changes --no-restore
• changelog fragment validation — 48 fragments
• CodeIndex workspace check — 1,014/1,014 files matched
• Codex adversarial review: first-round P2 fixed; second round found no actionable regression

Documentation and changelog

• updated the English and Japanese sections of TESTING_GUIDE.md
• added changelog.d/unreleased/4798.fixed.md

Follow-up candidates

None.

Fixes #4798